The invention relates to a process as well as to a device for a hydraulic mass-drive, especially for the closing and opening of the clamping mechanism in injection molding machines, with a controllable pressure-operated system.
The hydraulic drive is essentially based on the peculiarity of a liquid. The rough definition of a liquid is: A media in liquid state, which differs from others by the free movement of the constituent molecules and the low compressibility. In contrast to pneumatic controls, hydraulics are differentiated in that in praxis the hydraulic media is considered noncompressible, which represents a simplification for the mathematical evaluation and the design of a complete hydraulic system.
One of the tasks in injection molding and die-cast machines is to close and open the clamping unit. Since this movement has no direct influence on the process it should happen as fast as possible to improve productivity. This means a heavy mass (all moving parts of the clamping unit including the movable half of the mold) needs to be moved from point A to point B and returned by oil-hydraulics. The respective end positions are to be set with high accuracy.
For example, the German Patent No. 2 902 264 (corresponding to U.S. Pat. No. 4,375,747) proposes therefore to achieve the acceleration and deceleration of the clamp drive mechanism by a defined increase and decrease of the oil flow. By the use of several electronic control elements it is possible to avoid acceleration shocks. In detail a 4-way valve with 3 positions is arranged between pump and motor which can be controlled electrically and which has a so-called ramp assigned in the electrical control arrangement. The output signal of the ramp is amplified to control the directional valve (4-way valve). There is no consideration of any compressibility of the hydraulic fluid or any expansion of the drive components with increasing pressure in this case.
In die-cast and injection molding machines a resting or moving mass is driven by a plunger-cylinder unit, and is brought up to a certain velocity. For this purpose a throttle is increasingly opened to raise the acting pressure on the plunger, which drives the mass. The hydraulic fluid is not non-compressible, and also the components of the drive mechanism expand, especially the hydraulic fluid conductors, and therefore a certain part of the hydraulic flow input is absorbed capacitively by the drive system and does not cause a movement of the mass immediately. This capacitively absorbed hydraulic flow represents a compressed spring, which together with the mass is an oscillating device. The oscillating impulse increases with decreasing time for reaching the end velocity. Therefore, if one uses the set-point generator with a steep ramp between the various set-points, the system will respond with lowly damped oscillations, which means the desired velocity will only be reached gradually.
The best known method uses directional valves, which feed oil flow in the desired direction to a cylinder. These can only be used up to a certain velocity, since the acceleration is not controllable. More recent methods use so-called proportional- directional-valves. These control the valve opening in relation to the solenoid coil current. Even with this method the acceleration control is limited. In machine tools the typical acceleration period is approximately 6 times the swing period of the mass/spring oscillators (spring=elastic oil column), in order to reduce the time for operational movements. Typically, dependent on degree of dampening applied, vibration can occur already if the acceleration period is 3 times the swing period. Typical natural frequency for the clamping mechanism of a fully hydraulic injection molding machine, dependent on machine size is 3 to 8 Hz, which means usable acceleration ramps are in the order of 0.3 to 1.0 seconds. This results in a total loss time of 1 to 2 seconds per cycle.
According to the proposal of European patent No. 97275 (corresponding to U.S. Pat. No. 4,563,939) the inventors tried to improve the dynamic of the hydrostatic drive for die-cast or injection molding machines, in order to reduce the machine cycle time and/or to reduce the wear and tear of the machines. In this case the mass to be accelerated or decelerated is intermittently driven by hydrostatics with a correspondingly controlled valve arrangement, in which dependent on the direction of a change in a ramp angle, the oil flow increase or decrease is related to the compression volume of the system. The valve arrangement accordingly is controlled by a set-point generator, which provides a position or time dependent ramp with superimposed positive control impulse at the start of the ramp rise, or at the end of the ramp fall, respectively, and with a superimposed negative control impulse at the end of the ramp rise, or at the start of the ramp fall, respectively. This provided reasonable results in practical applications but in many cases, because of improper settings, improvement of the dynamic behavior is not achievable.
The objective of the invention is to reliably control the dynamic behavior with simple means, also at shortest acceleration periods, in order to reduce especially the machine cycle time.